1. Technical Field
The disclosed subject matter relates to an absolute rotary encoder, and in particular, to an optical absolute rotary encoder for detecting a rotational position as an absolute position.
2. Description of the Related Art
Known optical absolute rotary encoders disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 06-347292 can detect a rotational position as an absolute value based on transmitted light from a light emitting device. One exemplary structure of a rotary encoder is shown in FIG. 1. The rotary encoder includes a light emitting device 102, a drive circuit 101 for driving the light emitting device 102, a collimator lens 103 for collimating light emitted from the light emitting device 102, and a rotary disk 104. The rotary disk 104 has a recording surface including a plurality of tracks for allowing rotational position information to be recorded thereon as slits that represent a code pattern such as a pure binary code, a gray code, or other codes.
FIG. 2 shows one exemplary construction of the rotary disk 104. The rotary disk 104 has an optical recording surface including five tracks 104a through 104e. The outermost track 104a is used for forming slits representing the least significant bit (LSB) signal of rotational position information. The inner tracks are used for forming slits representing upper bit signals, and the innermost track 104e is used for forming slits representing the most significant bit (MSB) signal.
The rotary encoder further includes a rotary shaft 105, light receiving devices (being photodetectors) 106a through 106e, and a decoder circuit 107. The rotary shaft 105 is coupled to the rotational center of the rotary disk 104 to transmit externally provided rotational force to the rotary disk 104. The light receiving devices 106a through 106e receive the transmitted light through slits of the respective tracks 104a through 104e of the rotary disk 104 for photoelectric conversion. The decoder circuit 107 receives the output signals from the light receiving devices 106a through 106e in order to determine the absolute value of a rotational position.
In this rotary encoder, the light emitted from the light emitting device 102 is collimated by the collimator lens 103 such that it is linearly projected on the rotary disk 104 in the radial direction. Then, the light beams passing through the slits of the respective tracks 104a through 104e are received by the respective light receiving devices 106a through 106e for photoelectric conversion. The decoder circuit 107 can provide the absolute value of a rotational position by determining whether the transmitted light exists or not based on the output signals from the light receiving devices 106a through 106e. 
In the conventional optical absolute rotary encoder as described above, the light emitting device 102 and the light receiving devices 106a through 106e must be arranged at the respective sides of the rotary disk 104. Therefore, the light emitting device and the associated parts such as optical parts and electrical circuits (including a driving circuit) are disposed at one side of the rotary disk 104, and the light receiving devices and the associated parts such as optical parts and electrical circuits (including the decoder circuits and a driving circuit) are disposed at the other side of the rotary disk 104. This may complicate the structure for electrically connecting the parts with each other and for optical arrangement, resulting in difficulty in miniaturizing the entire rotary encoder.
In the meantime, the absolute pattern recorded on the rotary disk 104 of the above described rotary encoder is of a multi-track type for obtaining n-bit value from n tracks (the shown example is the case where n=5). In contrast to this, another type of optical absolute rotary encoder has been proposed which uses a single-track type absolute pattern for providing n-bit value by means of a single track (see, for example, Japanese Patent Application Laid-Open No. Hei 06-347288 (Japanese Patent No. 3093924), and Japanese Patent Application Laid-Open No. 2000-146623). This single-track type absolute pattern employs M-series codes, and the slits are recorded in accordance with the M-series code arrangement. In this code arrangement, the values represented by consecutive n-bit codes on a single track are different at every position. Accordingly, the correspondence between the rotational position and the value represented by the absolute pattern is stored in advance, and an absolute value of a rotational position can be determined based on the light pattern transmitted through the slits with reference to the stored correspondence.
The M-series codes can be generated using a shift register code generator and a primitive polynomial. For example, when n=5, the primitive polynomial can be represented by the following equation (1):f(x)=x5+x3+1  (1)
In this instance, the shift register code generator can generate M-series code pattern by allowing values from registers corresponding to the positions determined in accordance with the equation (1) to pass through an EXCLUSIVE-OR gate. The M-series code pattern thus generated is as follows:
                              00001                                      00010                                      00100                                      01001                                      …                                      10101                      }    ⁢  M  ⁢      -    ⁢  series  ⁢          ⁢      (          5      ⁢                          ⁢      bits        )    ⁢      :    ⁢          ⁢  00001001011001111100011011110101
If 5 consecutive bits are picked up, the 5-bit code can be uniquely determined in accordance with the position at which the consecutive 5 bits are detected. In this manner, the determined 5-bit code can provide the corresponding absolute position.
FIG. 3 shows another rotary disk 111 having a single track 111a for recording M-series absolute pattern when n=5. The optical absolute rotary encoder including this rotary disk 111 has a single light emitting device arranged at one side of the rotary disk 111 to project light along the circumference of the rotary disk 111. In this rotary encoder, the light transmitted through slits of the track 111a of the rotary disk 111 is received by five light receiving devices 112a through 112e arranged at the other side of the rotary disk 111 for photoelectric conversion. Then, a decoder circuit can detect whether the transmitted light exists or not based on the delivered signals, thereby determining the corresponding absolute value of a rotational position. In FIG. 3, dotted lines show the respective positions of the five light receiving devices 112a through 112e. This type of rotary encoder can determine the absolute value of a rotational position by means of a single track, and therefore it is easy to miniaturize the rotary disk 111 in comparison with the multi-track type device.
The optical absolute rotary encoder in accordance with the above-described system uses an M-series code pattern wherein the values represented by consecutive n bits on a single track are different from each other at every position. In this instance, a changed bit number of code pattern is complicated at a transition point of adjacent absolute position information. In other words, a plurality of bits change at the transition point and the timing of change may vary. In some cases, different code patterns which should not be derived from the corresponding position may appear. This may result in erroneously reading absolute position information during rotation. In order to more definitely determine such intermediate rotational positions, it is required to provide a certain mechanism for preventing reading error, such as a mechanical device or an auxiliary track.
In addition to this, in the conventional optical absolute rotary encoder using a single track system as described above, the light emitting device and the light receiving devices must be arranged at the respective sides of the rotary disk. This may complicate the structure for electrically connecting the parts with each other and for optical arrangement, resulting in difficulty in miniaturizing the entire rotary encoder.